Stop down structure for facsimile zoom lens

Abstract

A diaphragm is mounted for axial movement with the rear lens group in the zoom lens of a facsimile apparatus, and a closing ring axially movable with the rear lens group is rotatable via camming surfaces in a surrounding cylinder to close or open the aperture in accordance with the lens group axial movement.

What is claimed is: 
     
       1. In a variable image magnification, line-scanning type facsimile apparatus of the type having a zoom lens with at least one lens movable along an optical axis to vary the image magnification, said zoom lens having a numerical aperture, and a line sensor for receiving light through said zoom lens, the improvement comprising correction means for correcting the numerical aperture of said zoom lens by an amount corresponding to the position of said at least one lens along said optical axis, said correction means adjusting said numerical aperture in accordance with the equation:   F.sub.∞ (1+m/φ)=K     where F.sub.∞ is the F number of said zoom lens for an object at an infinite distance, m is the lateral magnification of said zoom lens, m>0, φ is the pupil magnification of said zoom lens, and K is a constant, thereby maintain substantially constant the optical energy received by said line sensor for a substantially constant main scanning rate and an auxiliary scanning rate proportional to 1/m.   
     
     
       2. A variable image magnification facsimile apparatus as claimed in claim 7, wherein said correction means, comprises: a rotatable closing ring;   a diaphragm having an aperture which varies in accordance with rotation of said closing ring; and   rotating means for rotating said closing ring in accordance with axial movement of said at least one lens.   
     
     
       3. A variable magnification facsimile apparatus as claimed in claim 2, wherein said closing ring is mounted for axial movement with said at least one lens, and wherein said rotating means comprises: first camming surfaces formed in a stationary member and extending at an angle with respect to said optical axis;   first cam follower means coupled to said closing ring and engaging said camming surfaces.   
     
     
       4. A variable magnification facsimile apparatus as claimed in claim 3, wherein: said stationary member comprises a stationary cylinder having an inner surface;   said at least one lens is mounted on a frame member slidably supported on said inner surface; and   said lens system further comprises second cam follower members fixed to said frame member and extending outside of said stationary cylinder through grooves formed in said cylinder extending substantially along said optical axis, and a rotatable second cylinder disposed outside of said stationary cylinder and having second camming surfaces in the form of grooves therein extending obliquely with respect to said optical axis, said second cam follower members engaging said obliquely extending grooves;   whereby rotation of said second cylinder will impart axial movement to said frame member, said closing ring will move axially with said frame member, and said closing ring will be rotated by said first cam follower means and camming surfaces.   
     
     
       5. A variable magnification facsimile apparatus as claimed in any one of claims 2-4, wherein said at least one lens comprises first and second lens groups each including at least one lens and said diaphragm aperture is variable in accordance with axial movement of said rear lens group.